The UN increased pressure on Iran yesterday to divulge more details of its nuclear programmes in order to establish whether Tehran is engaged in a clandestine quest for a nuclear bomb.

At the end of two days of debate in Vienna, the 35-member board of the International Atomic Energy Agency, the UN's nuclear watchdog, demanded that Iran answer questions on its nuclear policies, and grant greater access to IAEA inspectors.

But the statement by the board and remarks by the IAEA chief, Mohamed ElBaradei, stopped short of American demands for an ultimatum to Iran. On Wednesday President George Bush said for the first time that the US would "not tolerate" Iran's possession of a nuclear bomb.

At the meeting in Vienna, the US, backed by Britain, Canada, and Australia, pushed for a toughly worded resolution condemning Iran and insisted on unconditional and comprehensive inspections of Iran's nuclear facilities by the IAEA.

Such a resolution, which would have been "contentious and divisive" according to diplomats in Vienna, required a majority of the 35 states represented voting in favour. The US could not muster the majority and backed down. Instead Mr ElBaradei issued the statement reflecting the consensus at the meeting.

While calling on Iran to sign an "additional protocol" with the IAEA, which would afford the nuclear inspectors greater rights for snap inspections of suspect sites, the statement was milder than that sought by the US and avoided a split in the IAEA's board.

Washington insists that Iran is engaged in an illicit bomb-making programme. But the IAEA is unconvinced, although it is worried about Iran's failures to keep it posted on key elements in Tehran's nuclear projects.

Iran failed to report the import of a small amount of uranium from China 12 years ago. It has converted a small part of the uranium into uranium metal, which is not needed in civilian nuclear power, but is useful for weapons development. Iran is building a highly ambitious and sophisticated system of tens of thousands of centrifuges which could enrich uranium to the extent needed for a weapon. It is also building a heavy water power plant at Arak that can generate weapons-grade plutonium, but which appears to contradict its declared peaceful nuclear power aims.

Officials have blocked UN inspectors from a Tehran electricity company which has been making and testing centrifuge components, reportedly using enriched uranium to test the machines. The IAEA's statement called on Iran to suspend its plans to enrich uranium at the centrifuge complex until it was clear for what the enriched uranium would be used. The statement said Tehran should "grant the agency all access deemed necessary".

If granted, such access would go beyond the scrutiny that Iran is currently obliged to allow under the nuclear non-proliferation treaty.

IAEA officials said their inspections in Iran are a "work in progress" and were worried that a US ultimatum would have jeopardised their chances of maintaining access in Iran.

Diplomatic sources and arms control experts in Vienna believe that Iran is five years away from possessing a nuclear bomb, although a Russian arms control thinktank's analysis last week concluded it could have nuclear-tipped missiles by 2006.

The sources also did not discount the possibility of selective American air strikes on Iran's nuclear facilities, as implied by Mr Bush's assertion that Washington could pre-empt Iran possessing a nuclear bomb.

Any such strike would lead to an international furore. Russia, in particular, would be incensed, since it is the sole supplier of nuclear technology and expertise to Iran and has $800m (£480m) at stake in the nuclear power plant it is building at Bushehr in southern Iran.

Iranian and Russian officials yesterday voiced satisfaction with the results of the Vienna meeting, although the Americans are now pushing for the IAEA to issue a further report on Iran earlier than the scheduled date of September.